In an optical disc apparatus, an angle by which the data surface of an optical disc is out of alignment with a direction perpendicular to the optical axis of a light beam irradiated onto the optical disc is called a tilt angle. When the tilt angle is not zero, we say that a tilt has occurred. As optical discs have been developed to have high density, degradation of jitters in a recording signal or a reproduction signal becomes significant due to tilts occurring with optical discs and optical disc apparatuses, and it is getting difficult to maintain a sufficient level of performance.
FIG. 7 is a schematic diagram showing how the distribution of the strength of a light beam projected on the data surface of an optical disc changes due to occurrence of a tilt. FIG. 8 are characteristic charts showing a tilt to occur in relation to the jitter in reproduction signals and the error ratio. As can be appreciated from FIG. 7 and FIG. 8, a coma aberration occurs due to a tilt, and as a result, the jitter in the reproduction signals is degraded and also the error ratio increases.
When an aberration that occurs due to a tilt exceeds a tolerance value, problems arise where it becomes impossible to perform recording or reproduction under an optimal condition, and also the reliability of the data is lowered. In addition, the higher the recording density of an optical disc is, the smaller the tolerance range for the aberration caused by tilts becomes. Thus, it becomes difficult to secure a recording/reproduction performance of the apparatus.
In order to secure a recording/reproduction performance for a high-density optical disc, in a conventional disc apparatus, not only is the mechanical positioning of the optical systems and the driving systems adjusted, but also a tilt is detected during the operation of the apparatus and a tilt control is introduced in order to tilt an objective lens based on the detected result. As a result, a coma aberration is corrected and an optimal recording and reproduction of the apparatus is realized (see, for example, Reference 1).
Hereinafter, a structure of a conventional optical disc apparatus will be described.
FIG. 9 is a block diagram showing a structure of a conventional optical disc apparatus.
In FIG. 9, an optical head 100 includes a light source 101, a collimator lens 102, a polarized beam splitter 103, a quarter wavelength plate 104, an objective lens 105, a converging lens 107, a detector 108 and a focus actuator 113.
The light source 101 emits a light beam toward the data surface of an optical disc 106. The light source 101 is, for example, a semiconductor laser device.
The collimator lens 102 converts the light beam (diverging light) emitted from the light source 101 into a collimated light.
The polarized beam splitter 103 is an optical device for totally reflecting a linearly polarized light of the light beam emitted from the light source 101 and totally transmitting a linearly polarized beam which is present in the direction orthogonal to the linearly polarized beam of the light beam emitted from the light source 101.
The quarter wavelength plate 104 is an optical device for converting a polarized light of a transmitted light from a circular polarized light to a linearly polarized light or from a linearly polarized light to a circular polarized light.
The objective lens 105 converges a light beam onto the data surface of the optical disc 106.
The converging lens 107 converges the light beam that has passed through the polarized beam splitter 103 on the detector 108.
The detector 108 is a device converting a received light into an electric signal and includes a plurality of areas.
A preamplifier 109 is an electric device that converts a current output from each of the areas of the detector 108 into a voltage.
An FE signal generating unit 110 is an electric circuit generating, from a plurality of output signals from the preamplifier 109, a focus error signal (FE signal) that corresponds to a convergence state of a light beam on the data surface of the optical disc 106. A focus controlling unit 111 is a circuit outputting a focus controlling signal based on an output signal from the FE signal generating unit 110. A focus driving unit 112 is a circuit for outputting a focus actuator driving signal based on the focus controlling signal. The focus actuator 113 is an element moving the objective lens 105 in a direction perpendicular to the data surface of the optical disc 106 (hereinafter, this direction will be referred to as “the focus direction”). A tilt sensor 900 includes a light source 901 and a detector 902.
The light source 901 emits a light beam toward the data surface of the optical disc 106. The light source 901 is, for example, a light emitting diode.
The detector 902 is a device for receiving the light beam which has been emitted from the light source 901 and then reflected by the data surface of the optical disc 106 and converting the received light beam into an electric signal. The detector 902 includes a plurality of areas.
The tilt signal generating unit 903 is an electric circuit for generating a tilt signal that corresponds to a tilt of the optical axis with respect to the data surface of the optical disc 106, based on an output signal from the detector 902.
A tilt controlling unit 904 is a circuit for outputting a tilt controlling signal based on the tilt signal.
A tilt driving unit 905 is a circuit for outputting a tilt actuator driving signal based on the signal output from the tilt controlling unit 904.
A tilt actuator 906 is an element for tilting the optical axis of the light beam to be irradiated onto the optical disc 106 by tilting the optical head 100.
A focus controlling operation and a tilt controlling operation of the conventional optical disc structured described above will be described with reference to FIG. 9.
The linearly polarized light of the light beam emitted from the light source 101 is incident on the collimator lens 102 and converted into a collimated light by the collimator lens 102. The light beam collimated by the collimator lens 102 is incident on the polarized beam splitter 103. The light beam reflected by the polarized beam splitter 103 is converted into a circular polarized beam by the quarter wavelength plate 104. The light beam that has been converted into the circular polarized beam by the quarter wavelength plate 104 is incident on the objective lens 105 and is then converged onto the optical disc 106. The light beam is reflected by the optical disc 106, passes through the polarized beam splitter 103 and then is incident on the converging lens 107. The light beam that has been incident on the converging lens 107 is then incident on the detector 908. The light beam that has been incident on the detector 908 is converted into an electric signal in each of the areas of the detector 908. The electric signal converted in each of the areas of the optical detector 908 is then converted into a voltage by the preamplifier 909. A plurality of output signals from the preamplifier 909 are computed by the FE generating unit 110, so that an FE signal is output. The FE signal, which is the output signal from the FE generating unit 110, is input to the focus controlling unit 111 and passes through a phase compensating circuit and a low range compensating circuit that includes a digital filter using, for example, a digital signal processor (hereinafter, referred to as a “DSP”), so as to become a focus driving signal. The focus driving signal output from the focus controlling unit 111 is input to the focus driving unit 112, is amplified and is then output to the focus actuator 113. As a result of the operation described above, a focus control is realized where the convergence state of the light beam on the data surface of the optical disc 106 is controlled to be a predetermined convergence state at all times.
Next, a tilt controlling operation will be described.
The light beam emitted from the light source 901 toward the optical disc 106 is reflected by the optical disc 106 and is incident on the detector 902. The light beam incident on the detector 902 is converted into an electric signal in each of the areas of the detector 902. A plurality of output signals from the detector 902 are computed by the tilt signal generating unit 903 so that a tilt signal is output. The tilt signal, which is the output signal from the tilt signal generating unit 903, is input to the tilt controlling unit 904 and passes through a phase compensating circuit and a low range compensating circuit that includes a digital filter using a DSP, like in the focus controlling system, so as to become a tilt driving signal. The tilt driving signal output from the tilt controlling unit 904 is input to the tilt driving unit 905, is amplified and is then output to the tilt actuator 906. As a result of the operation described above, a tilt control is realized where the light beam is irradiated onto the data surface of the optical disc 106, as being orthogonal to the data surface at all times.    Reference 1: Japanese Laid-Open Publication No. 02-122432.